In recent years the art has given substantial consideration to the possibility of replacing the "standard" glass bottle with so-called plastic containers. In particular acrylonitrile containing polymers have been suggested for this purpose because of their useful barrier properties. Specifically, an acrylonitrile/styrene copolymer has been employed to form beverage containers capable of withstanding considerable internal gas pressure. Unfortunately some test studies indicate that acrylonitrile itself might be a carcinogen. Moreover when acrylonitrile monomer is present in the polymer, e.g., in copolymer pellets or an article or even initially in the polymer while in suspension or solution form, detectable, and perhaps significant, quantities of acrylonitrile and/or perhaps lower polymers of acrylonitrile may be leached out of the article during use. Concern exists, therefore, over the content of residual monomer, principally of acrylonitrile, in polymer articles or in the polymer even when still in solution or suspension form and specifically over the migration of monomer and lower polymer components from plastic bottles or other food containers into the food products contained in them.
Manifestly, the manufacturer of the polymers (homo or co-) and the plastic containers made therefrom employ polymerization and polymer treating techniques that are adapted to minimize the monomer content in the container walls. However, despite their best efforts to provide monomer-free polymer for the container forming operation and to avoid degradation of the polymer during the container forming operations, a residual acrylonitrile (monomer) content of 1 ppm-100 ppm in the container walls can almost always be found.
Any solution to the problem of monomer content in the container walls and migration of monomer from the container may not be divorcable from difficulties already known to the art. In particular, attention has been directed to the appearance of an extractable HCN content in packaging materials formed from nitrile polymers, including of course the acrylonitrile/styrene polymers, reference being made, in this regard, to U.S. Pat. No. 3,870,802. For example, containers believed to have been made according to practice of U.S. Pat. No. 3,870,802 (presumably from the polymeric nitrile resins described in that patent) tested out as having just less than 20 ppb (in the extract) of extractable HCN therein.
On the whole it is believed that as a practical matter, a monomer removing treatment should be carried out after the acrylonitrile polymer has been polymerized. Moreover, the treatment must avoid creating harmful side effects, such as for example creation of sufficient extractable HCN to affect end use of the polymer in food containers.